1. Field of the Invention
The present invention relates to a process for firing alloys containing easily oxidizable elements. More particularly, the present invention relates to a process for firing alloys containing easily oxidizable elements such as Al, lanthanides, rare earth elements and the like to obtain a sintered alloy of higher density superior in high-temperature oxidation resistance, etc.
2. Prior Art
In general, sintered alloys such as carbon steel, Fe-Cr stainless steel, Fe-Cr-Ni stainless steel and the like are obtained by firing in hydrogen, dissociated ammonia or vacuum.
As a case in which methane is added to the firing atmosphere, there is reported a process for firing a Fe-2Cu-C steel in N.sub.2 -(1-5)CO-(2-10)H.sub.2 -(0.25-0.5)CH.sub.4 (vol. %) ["Metals Handbook 9th. ed. vol. 7, Powder Metallurgy", pp. 308-376, Am. Soc. for Metals, (1984)].
It was found, however, that a powder of a metal (e.g. Fe-Cr-Al stainless steel) containing easily oxidizable elements, when fired in hydrogen, shows poor sinterability and is unable to achieve a desired density because the oxide film present on the powder particle surfaces, etc. cannot be removed by reduction. In some cases, the easily oxidizable components such as Al and the like are oxidized by a very small amount of oxygen or water contained in hydrogen and are consumed in the course of firing. In any of these cases, the sintered material obtained fails to express desired properties in oxidation resistance, etc.
When the process of conducting firing in N.sub.2 -(1-5)CO-(2-10)H.sub.2 -(0.25-0.5)CH.sub.4 (vol. %) is applied to a Fe-Cr-Al system, aluminum nitride is formed during firing because Al (easily oxidizable element) is reactive also with nitrogen, which prevents achievement of high density. This process is currently in practice in order to obtain a Fe-2Cu-C sintered alloy having a higher hardness at the surface by cementation, and pays no attention to high-temperature oxidation resistance, etc.
The present invention has been made in view of such a situation. The object of the present invention is to obtain a sintered alloy superior in high-temperature oxidation resistance, etc. by, in the process for firing alloys containing easily oxidizable elements, removing the oxidized, layer, the water and oxygen present on the material powder particle surfaces or in the atmosphere used and improving the sinterability of the powder particle surfaces.
As a result of extensive study, the present inventors found that the above object can be achieved by conducting firing in an atmosphere where an appropriate amount of at least one hydrocarbon gas is added to hydrogen gas. The finding has led to the completion of the present invention.